Laser Measures Shiny Objects in 3-D

OKAYAMA, Japan -- Laser rangefinders can be used to precisely determine the three-dimensional shapes of objects, but only if the objects display lambertian reflection. Now researchers at Okayama University have developed a system that can measure the shape of specular objects as well. Although it currently is too slow for production-line machine vision, the laser rangefinder may have applications in off-line measurements of precision-machined objects.

Researchers at Okayama University have adapted a triangulation-based laser rangefinder to perform three-dimensional shape measurements on specular objects, such as this gold cup. Courtesy of Mitsuru Baba.
Triangulation-based laser rangefinding techniques can fairly easily measure the three-dimensional shapes of objects with lambertian properties, explained Mitsuru Baba, a member of the research team. Rangefinders scan a light stripe over such objects and collect the resulting image. Because there are no shiny surfaces, the reflected light diffuses hemispherically and forms a complete image on a sensor.

For shiny objects, however, the reflected light is concentrated in a small region around the specular direction. The light from two points on the object can strike the sensor at the same spot, which confuses triangulation.

To overcome this problem, the research team designed a lens-and-mask system to fit a standard laser rangefinder. The horizontal slit covers the lens and limits the light path in the vertical direction. A series of thin plates mounted vertically between the lens and a series of linear CCDs limit the light path in the horizontal direction.

Unlike the general collection method of a traditional rangefinder, only the light stripes that can pass through the mask system reach the sensor in this setup. To complete the scan of an object, the projection angle and the position, or varying combinations of both, must be adjusted until the light stripe strikes the detector.

Need for speed

The researchers tested the system by measuring the 3-D shapes of several objects with lambertian and specular surfaces, including a plane, a cylindrical column, and an object with convex and concave surfaces. Baba said, "The experimental results demonstrate that the system can accurately measure objects with lambertian and specular surfaces, if the change of shapes is relatively small."

He said the maximum error of the system is roughly 150 µm, which corresponds to a relative error of 0.1 percent. The process is slow, however, requiring 20 minutes to acquire 100 measurement points. Nevertheless, the system in its current state could be useful for determining the 3-D shapes of objects such as precision-machined tool-making dies. It also could be used inline as a screening test for objects that do not need detailed measurements.

Baba said that the group, which published its results in the January issue of Optical Engineering, is working to improve detection speed and to validate the practicality of applying the method to objects with a variety of reflective surfaces.